In the hot-rolling and cooling of steel rod (i.e. 7/32" up to 1/2" 0.D.), up until around 1964, the universal practise was to roll the rod, and, after cooling it with water in the delivery pipes, to coil it into a bundle either with or without forced air cooling. That method had serious disadvantages in that the physical properties of the steel could not be controlled and scale (oxidation) losses were significant. In general, medium-to-high carbon content (i.e. 0.3% C. to 0.9% C.) steel rod processed by that method required heat treatment (called "patenting") prior to being drawn into wire. That process produced steel rod in the low carbon content range (i.e. 0.03% C. to 0.20% C.) which, in some instances, could be processed to finished product without heat treatment, but in many other instances, required annealing or similar treatment. In the low alloy and high alloy content grades, further heat treatment was invariably required.
The advent of the so-called Stelmor process in 1964 (U.S. Pat. Nos. 3,231,432, 3,320,101, and 3,390,871), caused a major change in industry because it permitted rod to be rolled, laid, cooled and collected in such a way that, in the medium-to-high carbon content grades, the rod could be further processed in many instances to finished product without requiring any heat treatment. This was accomplished by first water-cooling the rod in the delivery pipes to about 800.degree. C., then laying it in spread-out ring form onto a moving conveyor, and cooling it through transformation under the influence of an air blast passing through both the conveyor and the rings. The Stelmor process went into wide-spread use and rapidly rendered the prior method virtually totally obsolete.
Although the Stelmor process made tremendous savings for high carbon content steel rod, it did little for low carbon other than scale saving. In fact, it tended to cool low carbon rod too rapidly thereby rendering its tensile strength too high (and hence its ductility too low) for many uses. As a result, the prior practices of annealing low carbon rod continued more or less without change even after the advent of the Stelmor process. However, since low carbon steel rod represents about 75% to 80% of the demand for steel rod, and annealing costs are very substantial, a search still continued for ways to adapt the Stelmor equipment for the slow cooling requirements of low carbon rod. In addition, along with the rise in demand for products employing alloy constituents as in steel belted radial automobile tires, special reinforcing, and welding rods, significant tonnages of low alloy steel rod began to be rolled. Such steels require extremely slow cooling, likewise not hitherto feasible with equipment of the Stelmor type equipped for rapid cooling.
One partial solution of the low carbon problem was to have one rod rolling facility equipped for rapid, Stelmor-type cooling, for use with high carbon rod, and additional installations equipped to roll low carbon rod and cool it slowly or even partially to anneal it as described in U.S. Pat. No. 3,711,338. As mill delivery speeds began to increase into the +20,000 fpm range, however, it became even more desireable to provide more versatility in a single installation so that the advantages of high speed rolling could be attained and at the same time provide optimum processing conditions for the entire range of steel rod products.
An early attempt at versatility was practised in Holland in the late 1960's. It employed pivotally mounted, removable, insulated covers over a typical Stelmor bar and chain type conveyor. Also when slow cooling was desired, transite panels were inserted between the bars on the conveyor. That installation was only partially successful. Cooling rates within a single coil from about 0.5.degree. C./sec to 2.degree. C./sec were achieved. The slow cooling was not sufficiently uniform, however, for most products and the process was not adopted commercially.
Another attempt at versatility is described in U.S. Pat. No. 3,711,338 in which a roller hearth furnace is positioned alongside a typical Stelmor installation with provision to move the Stelmor conveyor aside and the furnace into line with the rolling mill so that very rapid cooling sufficient partially to form martensite can be performed initially and then followed by an annealing (or martempering), type of treatment.
Still another design for versatility is described in U.S. Pat. No. 3,930,900 in which radiant heating elements carried by removable, pivotally mounted covers are used to retard the cooling rate. This equipment performed well on some products. Additional designs for versatility are disclosed in U.S. Pat. No. 4,242,153 which offers the options of batch austempering, martempering and annealing in parallel with Stelmor.
In addition, recent discoveries in both the slow cooling and rapid cooling modes have shown the desirability of adding additional processing options to both the slow and rapid cooling modes.
For example, in order to achieve uniformity in the slow cooling mode, a procedure called "IRC" (intermittent reheat cooling) is desireable. IRC is described in copending application Ser. No. 215,331 (12/11/80) (see also European patent application Ser. No. 81300094.0). It involves allowing the rod to cool for a measured period of time under insulated "hot-box" conditions, and then reversing the direction of heat flow by passing the rings through a zone in which high heat is applied to the rod rings, as in a furnace, from underneath and above. In this way, the exposed, rapidly cooled places are reheated more rapidly, the reverse of the manner in which they had been cooled more rapidly previously, and thereby the temperature differences are equalized. The intermittent high heat applications, of course, are gradually diminished to achieve a gradual uniform overall cooling. They can, however, be maintained if tempering or annealing is desired.
Recent discoveries in the rapid cooling mode show that rod having properties approaching those of lead patented rod can be made if the rod is laid onto a relatively cool conveyor at relatively high temperature (at which austenite grain growth is rapid) and forced-air is applied to all parts of the rod gradually at first, building up to a maximum intensity during transformation. This procedure is also described in copending application Ser. No. 215,331 (12/11/80).
A basic object of this invention therefore, is to provide, in one and the same piece of equipment, a maximum range of treatment options and an ability for changes from any option to any other with a minimum of inconvenience. More specifically, an object of the invention is to provide equipment adapted for slow cooling, which offers the options of IRC, annealing, austempering, martempering and the like with or without preliminary water cooling and also adapted for rapid cooling together with means for maintaining a relatively cool conveyor and providing a greater intensity and more effective air cooling flow than was available in prior equipments. A further object is to provide the foregoing together with push-button control for rapid change from any one option to any other.